There will be presidential elections in Russia in March 2018. EU and USA do not considered Crimea as part of Russia, however citizens of that region (around 2 million people) will take part in that vote. Does it mean that the EU and USA can't legally consider those elections legitimate?
First of all, there's a difference between recognizing that Crimea is a Russian territory and recognizing that Crimean residents are Russian citizens. Taking up the Russian citizenship was voluntary and the EU or the US cannot dictate whether or not a given person can become a Russian citizen by choice. There are still hundreds of thousands of dual Ukrainian-Russian citizens in Crimea and thousands others who refused to become a Russian citizen in the first place. Likewise international organizations cannot object to countries giving out their citizenship to residents of a certain territory, like in the case of Austria giving out their passports to residents of South Tyrol.
Second, there are even countries where non-citizens are eligible to vote for Parliament. E.g. in the UK you can participate in the general election (or stand as a candidate) if:
To vote at the UK general election you must be registered to vote and:
- be a British, Irish or qualifying Commonwealth citizen
And I haven't heard of anyone contesting the validity of British elections...
So to answer your question: no, Russian elections of any kind do not become illegitimate simply because Crimean residents can participate in them.
You are asking a loaded question because I can predict what the outcome is.
If Russia denies the Crimeans to vote, then the EU/USA will vilify Russia because it suppresses the opinion of the Crimeans and it shows that Russia is afraid of a vote which will show that the Crimeans do not want to be ruled by Russia.
If Russia allows the Crimeans to vote, then the EU/USA will vilify Russia because it shows that Russia still legitimates the brutal annexion of Crimea and (as you imply) that the presidential election is doubtful/invalid. No word over the opinion of Crimeans anymore.
Either way, the Russians are the bad boys.
Short version: neither the US or the EU rule the world.
Long version: Legitimate is either an internal legal question, or an external political statement.
Internally, it only matters if there is also an internal mechanism for enforcing the law.
Externally, calling another government non-legitimate is part of the process of either putting pressure on that government or providing support to a competing authority. It gives the external government justification for it's actions.
I doubt any polity has laws requiring them to take specific actions when they declare another government non-legitimate or specific processes they are required to perform for all other governments to determine their legitimacy.
While not a lawyer, I think I would have heard of a law requiring the US to determine the legitimacy of every other nation on the planet.
Basically I believe they (US and EU) can "legally" recognize anyone that they want to as the legitimate government, practically this recognition is typically given to the forces in physical control of an area. Occasionally, they recognize someone else or no one for mostly internal political reasons. For example, the US recognized Taiwan as the sole, legitimate government of China... until recognizing the PRC was more valuable in the Cold War.